The present invention relates to electric vehicles and in particular to a electronic systems and methods for control of the vehicle.
Due to the basic characteristics of electric motors, it is beneficial for electric vehicles to be fitted with at least some form of multi-speed, automatic transmission which selects shift points to operate the electric motor or motors in a predetermined RPM range. The multi-speed transmission is very important because it is very inefficient to achieve enough torque from a cost effective drive system to accelerate quickly or climb a steep grade when a vehicle is operating at low speed, without the ability to operate at least one motor in its higher speed range.
Improved methods for selecting the shift points for a multi-speed, automatic transmission in an electric vehicle have been proposed by the inventor of the present invention, but these methods rely on parameters including accurate vehicle speed estimates. Known electric vehicles measure speed in various ways, generally using light or magnetism, to measure the speed of an exposed moving part of the drive train. The whole system, including the wiring, is exposed to the elements and can be easily damaged or knocked out of adjustment. It is also subject to failure due to dirt, water, mud and other debris or erroneous signals created from outside sources. The vehicle speed may obtained in various ways, but is generally obtained using measurements of light or magnetism, to obtain the speed of an exposed moving part of the drive train which is proportional to vehicle speed. The speed measurement system adds cost and weight, and the entire speed measurement system, including wiring, is often exposed to the elements and can be easily damaged or knocked out of adjustment. The speed measurement system is also subject to failure due to dirt, water, mud, or other debris and errors may be introduced by erroneous signals originating from outside sources.
Accordingly, the need exists for a method to determine the speed of the vehicle, which method is not vulnerable to the failures mentioned above.
Known electric vehicles further implement a power control operator interface, which determines the amount of power that will be applied to the drive wheel(s) of an electric vehicle and sometimes, a single braking control operator interface for both friction braking and regenerative braking. The power control operator interface, often called the “accelerator pedal”, “gas pedal”, or “throttle”, is commonly positioned as a right positioned pedal on the floorboard of the vehicle, or as a twist-grip on the right handlebar. Similarly, the braking control operator interface is commonly positioned to the left of the power control operator interface on the floorboard of the vehicle, or as a lever mounted on the handlebar and connected to mechanisms which apply the regenerative braking and friction braking to decelerate the vehicle.
Each of the operator control interfaces, for both the control of the application of power and regenerative braking, actuate a corresponding transducer which converts a mechanical force and/or displacement into an electrical output signal comprising a voltage, a resistance, an inductance, a current, a digital signal, or other means which varies as a function the mechanical position and/or displacement of the operator control interfaces. The electrical output signals are provided to the electronic controller as inputs to indicate the amount of power or regenerative braking the operator desires.
In mechanics, power is generally defined in terms of speed multiplied by force, or in the case of rotational motion, power is defined in terms rotational speed multiplied by torque. In electrical terms, power is defined in terms of current multiplied by the voltage. As an electric motor transforms electrical power into rotational mechanical power, the output shaft speed is about proportional to the applied voltage, and the output torque is about proportional to the applied current.
There are two common methods used to control the power applied to a drive system. The first method uses the signal generated by the power control operator interface described above, to create power from the motor controller to the motor(s), where the power is controlled by varying the duty cycle of the Pulse Width Modulated (PWM) power supplied to the motor to maintain a desired average voltage which is a function of the magnitude of the control signal. As a result, this will vary the speed of the motor output shaft, and resulting vehicle speed. Most controllers using this method of varying the output voltage to control the power will implement current limiting circuitry to protect the controller, motor, wiring and the mechanical drive train. The second method to control the power applied to the drive motors is also to vary the duty cycle of a Pulse Width Modulated (PWM) power supplied to the motor but to maintain a desired averaged current which is a function of the input control signal. As a result, this will vary the torque of the motor output shaft, and resulting vehicle driving force and acceleration. In this case, the maximum output voltage is only limited by the applied input voltage to the motor controller.
Each of the above methods to control an electric vehicle drive motor electrical power has its drawbacks. The first, using the power control operator interface to control the power signal provided to the motor by the electronic controller as a function of the applied voltage, has a tendency to not provide the amount of current required to obtain the acceleration the operator intends. Typically, when a Direct Current (DC) motor's full rated voltage is applied, it will draw about ten times the current at an initial startup condition compared to when operating at its rated power level. The factors which limit the current applied to the motor, are the capacity of the power source (i.e. a battery), the limits of the electronic controller, the line resistance, and the motor internal impedance. Therefore the current could quickly reach the limit of the electronic controller with only partial application of the power control operator interface, resulting in excessive torque and higher acceleration than desired by the operator. This could result in a momentary unsafe level of acceleration, excessive battery drain, and less vehicle range. This is especially true for higher quality motors with minimal internal impedance.
The second method of controlling the power supplied to the motor, alleviates the draw backs mentioned above, but does not properly regulate the speed of the drive wheels which diminishes operator control. For example, under low traction conditions such as sand, water, snow or ice, the drive wheels could quickly spin up to maximum speed with little operator input, this can result in safety hazards.
Further, due to the basic characteristics of electric motors it is beneficial for electric vehicles to be fitted with at least some form of multi-speed, automatic transmission which selects shift points to operate the electric motor or motors in their most efficient RPM range. But when a multi-speed automatic transmission is applied to an electric vehicle, the power control criteria during the shifting process is almost completely different than when applied an Internal Combustion Engine (ICE) vehicle. In this case, many problems arise using either of the existing control methods mentioned above. For example, when using the voltage based power controller, the operator would not know where or when to reposition the operator interface(s) to maintain a desired rate of speed and acceleration, as the transmission is shifted between gears. As a result, the shifting will not be smooth, excessive torque will be transmitted to the drivetrain, the vehicle will accelerate at a rate other than what is desired, and may quickly achieve a speed beyond the intended speed of the operator.
For any given gear ratio, it is most efficient to allow the motor to reach an RPM beyond the peak efficiency before shifting to the next gear ratio. Thus, in order to achieve a desired speed and acceleration of the vehicle, the operator will have to adjust the position of the power control operator interface to compensate for the current vehicle gear ratio. As a result, to accelerate up to the shift point in any given gear, the operator is required to apply nearly full application of the power control operator interface to reach the high motor speed required. Once the vehicle has reached the predetermined speed where it shifts to the next gear, the position of the power control operator interface would need to be repositioned to limit the speed and torque of the motor. Because of the excessive application of power to the motor at the time of shifting, the shifts would not be smooth, a high impulse of torque will be transferred through the drive train causing greater wear and possible failure of the system, the motor will operate at a less efficient level, and it will be more difficult to maintain the desired speed. Unfortunately, the human operator would not be able to adequately perform the operation as described above causing fatigue to the operator, rough shifts, reduced drivetrain life, and possible safety hazards.
Additionally, most known electric vehicle motor controllers implement a “soft start” feature which gradually applies the output power to the motor as the operator input signal is increased. The intent is to make a smooth application of power, rather than allowing a rapid surge of current. The problem that this creates is if the motor speed is not monitored, when the vehicle is already in motion, and the power control operator interface is released and the subsequently re-applied, it may require excessive time until the voltage applied from the motor controller reaches the Back Electromotive Force (BEMF) of the motor at this speed. Until the applied voltage exceeds the motor BEMF no current will flow, and therefore no power will be applied to the drive train.
Similarly, on a vehicle equipped with regenerative braking, when the operator applies the brakes, using the braking control operator interface, producing a similar operator control signal as described above, if a constant load is placed on the drive motor(s), which are now serving as generator(s) to recharge the batteries, as the vehicle slows, less current will be generated, and consequentially there will be less deceleration. In this case, the operator will need to vary the brake position to maintain a desired rate of deceleration. On a vehicle with a multi-speed automatic transmission having predetermined shift points, the problem is compounded as the transmission downshifts and a constant load is maintained on the motor terminals, there will be marked differences in the rate of deceleration as the transmission downshifts to a lower gear ratio. Even when a system implements a constant current output during regenerative braking, the problem will still be present during the downshifts.
Accordingly, there exists the need for an intelligent operator interface for applying both power and braking, controlling both the voltage and current applied to the drive motors which mimics the position of the power control operator interface and the braking control operator interface and does not require the operator to modulate his input based on the selection of varying gear ratios. In addition, there exists the need to reduce the delay between when the operator actuates the power control operator interface and when power is applied to the drivetrain.
Due to the basic characteristics of electric motors, it is beneficial for electric vehicles to be fitted with at least some form of multi-speed automatic transmission which selects shift points to operate the electric motor or motors in a predetermined RPM range. The multi-speed transmission is important in electric vehicles because it is very inefficient to attempt to obtain enough torque from the electric motor to accelerate quickly or climb a steep grade while the electric motor is not in its efficient RPM range, for example, at low vehicle speed with high gearing.
Shifting a multi-speed transmission based on vehicle speed alone does not guarantee an optimum result. The vehicle speed at which the actual optimum shift points occur often vary depending on different vehicle operating conditions and operator inputs. For example, if the vehicle is carrying a heavier payload, experiencing increased external drag, or climbing a steep hill, and the transmission shifts to the next higher gear at a programmed vehicle speed selected for level conditions, motor speed may drop enough to cause the transmission to shift back to the previous lower gear. This often results in the transmission shifting back and forth (i.e., searching) between gears which causes a loss in overall vehicle range, increased wear on parts, and annoys the operator.
Known electric vehicles measure speed in various ways, generally using systems including light or magnetic sensors, to measure the speed of an exposed moving part of the drive train. Such systems, including the sensors and associated wiring, are exposed to the elements and can easily be damaged or knocked out of adjustment. The systems are also subject to failure due to dirt, water, mud and other debris or erroneous signals created from outside sources. Further, the sensors only measure the vehicle's forward speed, and do not measure acceleration or the incline of a road the vehicle is traveling on.
Accordingly, the need remains for a method and apparatus to determine the speed and acceleration of the vehicle, and incline of the road surface, for selection of efficient shift points, which apparatus is not vulnerable to commonly encountered hazards.